1. Field of the Invention
The present invention relates to a method for the surface treatment of a magnetic amorphous alloy. More particularly, the present invention relates to a method for forming on a magnetic amorphous alloy a film which is insulating and corrosion-resistant and, especially, can prevent secular deterioration of the magnetic properties due to chemical and physical changing of the surface properties of the magnetic amorphous alloy.
The amorphous alloy herein is a metal which has a random atomic arrangement like that of liquid and can be produced by supercooling the molten metal on a cooled substrate. The magnetic amorphous alloy comprises a metallic component(s) such as Fe, Co, and/or Ni in a total atomic amount of from 70% to 88%, a metalloid component of from 7% to 25% of B, and, occasionally, a balance of Si, P, and/or C. The magnetic amorphous alloy may further contain up to 5% of Cr, Mo, Nb, and/or V.
Since the production of a magnetic amorphous alloy is simple, as is described above, and since a magnetic amorphous alloy exhibits numerous properties superior to those of a magnetic crystalline alloys, magnetic amorphous alloys has received attention concerning the practical application thereof. Especially, the magnetic properties of an amorphous alloys, are superior to those of the materials currently being used. For example, a magnetic amorphous alloy exhibits less of a watt loss than does a grain-oriented silicon steel sheet by one or more orders of magnitude and also exhibits more of a permeability and magnetic flux density than do Permalloy and ferrite, respectively.
2. Description of the Prior Art
A magnetic amorphous alloy exhibits a poor corrosion resistance, especially when Cr is not added thereto in order to attain an especially high magnetic flux density. In this case, rust spots form on the magnetic amorphous alloy sheet and impair the magnetic properties thereof. Also, thickness of the magnetic amorphous alloy sheet is increased on the portions thereof where rust forms. Due to the increase in the sheet thickness, the space factor of a core made of a magnetic amorphous alloy is lessened.
The present inventors investigated magnetic amorphous alloys and arrived at the following conclusion and the obtained the following results.
A. When a magnetic amorphous alloy is used as a soft magnetic material, an insulating coating is preferably applied on the surface of the magnetic amorphous alloy so as to reduce the eddy current loss. Since a magnetic amorphous alloy sheet is thin and has a rough surface, the space factor of a core made of a magnetic amorphous alloy sheet is low. Such low space factor may be drastically lessened unless the insulating coating of the magnetic amorphous alloy is dense and extremely thin.
B. When a magnetic amorphous alloy which is free of Cr and is not subjected to any surface treatment is exposed to the atmosphere in a room for a period of from ten days to one month, red rust spots form on the surface of the magnetic amorphous alloy. Consequently, the magnetic properties of the portions of the magnetic amorphous alloy where the rust spots formed are impaired and the sheet thickness of the magnetic amorphous alloy is locally increased, with the result that the space factor maybe lessened.
C. A magnetic amorphous alloy containing Cr has an enhanced corrosion resistance but exhibits too low a magnetic flux density to be used, particularly, as a wound core.
D. Metal oxide forms on the surface of a magnetic amorphous alloy during supercooling and solidifying of the Si-containing molten metal. The film of metal oxide contains especially SiO.sub.2, is thin and dense and is repellent to water and an aqueous solution. Therefore, even if such thin magnetic amorphous alloy sheet is subjected to a surface treatment with acidic solution having a pH of 0.5.about.5, used for conventional phosphating or the like, it is rather difficult to uniformly apply a coating with good wettability.
E. Nippon Steel Corporation filed Japanese Unexamined Patent Publication No. 59-25998, in which a film comprising (a) a chromium hydrate hydroxide, (b) hydrate chromium hydroxide plus chromium, (c) phosphoric acid plus (a) and/or (b), is disclosed as a coating of the magnetic amorphous alloy.
The disclosed film is formed by the dual stages, including the first rinsing step in the aqueous hydrofluoric acid solution. The film formation is desirably carried out by a singe-stage process.